COEN 350

Network Defense in DepthFirewalls

Terms of the Trade Border Router

First / last router under control of system administration.

DMZ Demilitarized zone. Security is low, since not

protected by firewall. Locate webservers and other services there that generate potentially unsafe traffic.

Firewall Filters packages based on a

variety of rules.

Terms of the Trade IDS

Intrusion Detection System. NIDS: glean intrusion

signatures from traffic. HIDS: monitor activity at a

host on which they are located.

VPN Virtual private network

Screened subnet Area protected by an internal

firewall.

Terms of the Trade Configuration Management

Known vulnerabilities account for most of actually perpetrated exploits.

For most of them, patches were available, but not installed.

CM tries to enforce uniform security policies.

Backdoors An entrance into the system

that avoids perimeter defenses.

Defense in Depth

Rule 1: Multitude of security measures. Do not relay on one security

mechanism. Rule 2: Do not make security so

expensive / burdensome that you give legitimate users an incentive to circumvent security.

Defense in Depth Example: External tcp packet passes:

Internet Perimeter Router Internet perimeter firewall DMZ firewall Network IPS NetFlow

Analyzes connections on network Antivirus on host Host IPS

Firewalls Firewalls are perimeter defense:

Keep the bad stuff outside, enjoy life inside.

Filtering Signature

Any distinctive characteristic that identifies something (with a high degree of probability)

Signature Types Atomic Signatures

Single packet, single event, single activity is examined.

Stateful Signatures State: Needed when analyzing multiple pieces

of information that are not available at the same time.

Filtering Atomic vs. Stateful Signatures

LAND attack Attacker sends TCP-SYN packet with same source

and destination address. Caused TCP stacks to crash.

Can be discovered looking at a single packet. Search for string “etc/password” in a URL

Attacker fragments the packet so that the string is not in either fragment.

State is needed in order to recognize the attack.

Filtering Signature Triggers

Pattern Detection Simple string search

Search for string “etc/passwords” ARP Protocol decoders search for string only in

protocol fields. ARP request with source address FF:FF:FF:FF:FF:FF

Anomaly Detection Traffic going to an unusual port. Protocol compliance for http traffic

Behavior Detection Abnormally large / small fragmented packets Search for RPC requests that do not initially utilize

the PortMapper

Filtering

Signature Actions Generating an alert Dropping / preventing an activity Logging the activity Resetting a TCP connection Blocking future activity Allow activity

Packet Filtering

Static Packet Filtering Allow or deny access to packets

based on internal characteristics.

access list 111 deny ip host 205.205.205.205.1 any

access list 111 permit tcp host 205.205.205.205.1 any

access list 111 deny icmp any any echo-request

access list 111 permit icmp any any packet-to-big

access list 111 deny icmp any any

Cisco extended ACL

Static Packet Filtering

Difficult to design efficient rules. Easy to get the rules tables wrong and allow

bad traffic. Security risks

People can piggy-back bad messages in harmless ones.

http traffic is known to be used as a backdoor. Loki uses unused fields in normal TCP packets.

Fragmentation allows the filter to look only at a fragment

Most only look at the first fragment

Static Packet Filtering

Configuring a packet filter: Security Policy: what is allowed, what

is not allowed. Allowable types of packets must be

specified logically, in terms of logical expression on packet fields.

Expressions need to be rewritten in the firewall vendor’s language.

Static Packet Filtering Example

Security Policy: Allow inbound mail messages (SMTP, port 25),

but only to gateway. Block host faucet.

action Our host

port Their host

port comment

block * * faucet * We don’t trust these people.

allow OUR-GW

25 * * Connection to our SMTP server

Static Packet Filtering Example

If no rule applies, then the packet is dropped. Without additional rules, our rule set would drop all

non-mail packets. There would also be no replies. Beware of a rule like this (intended to allow acks)

Based solely on outside host’s port number. Port 25 is usually the mail port. But there is no guarantee.

action Our host port Their host port comment

allow * * * 25 Connection to their SMTP

port

Static Packet Filtering Example

Expand rule set to allow connection with the outside:

action Our host port Their host port Flag comment

block * * faucet *

allow OUR-GW 25 * *

allow (our host) * * 25 Our packets to their port

allow * 25 * * ACK Their replies

Specify the names of all machines allowed to send mail to the outside here.

Static Packet Filtering

Combating Address Spoofing At a minimum:

Don’t allow inside source addresses coming in.

Don’t allow outside source addresses going out.

Block source routing at the border routers.

Static Packet Filtering Routing Information

If a node is unreachable from the outside then the node is almost (but not quite) as safe as a node disconnected from the net.

Internal routers should not advertise paths to such nodes to the outside.

Filter routes learned from the outside: Protects against subversion by route confusion. Route squatting:

Use internal addresses that belong to a different domain. The nodes are de facto unreachable from the outside. Use non-announced addresses. (e.g. 10.x.x.x)

But beware, when companies merge, these addresses tend to be incompatible.

So pick addresses in unpopular address ranges.

Static Packet Filtering Performance

Packet filtering is done at the border. No degradation for the internal network.

Typically, connection to ISP is the bottleneck.

However: Degradation depends on the number of

rules applied. Can be mitigated by careful ordering of

rules.

Application Level Filtering Packet filters only look at

The source address The destination address TCP / UDP port numbers TCP / UDP flags.

Application filters deals with the details of the service they are checking. E.g. a mail application filter looks at

RFC 822 headers. MIME attachments. Might identify virus infected attachments.

Application Level Filtering

Snort: Allows to set up rules that pass a

packet on to another service. Commercial firewalls

Include application level filters for many products.

Use non-disclosure agreement to obtain proprietary protocols

Dynamic Packet Filtering Stateful Firewall Still look at each packet. Maintains a state of each connection.

Implements connection filtering. Dynamically adjust a filtering table of current

connections. Implementation

Adjust the filtering rules dynamically. E.g.: We started an HTTP connection to a given host. Now HTTP packages from that host are allowed.

OR: Terminate the connection at the firewall and then have the firewall call the ultimate destination (proxying).

Proxy Firewalls Proxies act on behalf of a client. Proxy firewall

Reverse Proxy Receives packages on one card. Processes requests. Translates them into internal requests on other card. Receives answers from inside and translates to the

outside.

Proxy Firewalls Proxy firewall

Forward Proxy Receives requests from the

inside. Processes requests. Translates them into requests to

the outside on other card. Receives answers from outside

and translates to the inside. Acts on behalf of inside machine

that is protected from the vagaries of the internet.

Proxy Firewalls

Application level proxies work at the level of application.

Circuit-level proxies does not understand the application makes filtering decisions by validating

and monitoring sessions.

Application Inspection

Dynamic Firewalls allow selective inspection of applications: http ftp dns icmp …

Application Inspection

DNS example (Cisco ASA DNS inspection) Guarantees that the ID of the DNS

machine matches the ID of the DNS query

Allows translation of DNS packets using NAT

Reassembles DNS packets to verify its length.

Application Inspection

SMTP (Cisco ASA protection) Protects against SMTP-based attacks

by restricting the types of SMTP commands.

Illegal command is modified and forwarded.

Typically, receiver replies with an SMTP error 500 (command not recognized)

Checks size, …

Network Address Translation Originally designed to address the IPv4

address shortage: Use internal IP addresses

192.168.x.x 172.16.x.x, 172.32.x.x 10.x.x.x

NAT box is dual hosted: One connection to interior network Other connection to exterior network

with “overloaded” or “public” address

Network Address Translation Internal host initiates TCP connection to

the outside. NAT box takes TCP package, replaces

source IP with its public IP, port with a port chosen for that connection

When reply return to NAT box, forwards package to internal host.

NAT uses stored connection data to determine the interior address

Network Address Translation NAT increases security:

NAT hides host identities NAT hides weak sequence number

generating hosts by randomizing sequence numbers.

NAT disables inbound TCP connections NAT breaks most UDP protocols

NAT allow load balancing and transparent failover

Network Address Translation

Full Cone NAT (one to one NAT) all requests from same internal IP

address and port are mapped to the same external IP address and port.

Allows external host to send package to the host by using the mapped external address.

Network Address Translation

Restricted Cone NAT All requests from same internal IP

address and port are mapped to the same external IP address and port.

External host can only send package to internal host if the connection has already been established

Network Address Translation

Port Restricted Cone NAT Like restricted cone NAT,

but only for certain port numbers Symmetric NAT

All request from the same internal IP address and port to a specific destination IP and port are mapped to a unique external source IP address and port.

If the same hosts sends to the same port, but another IP address, then a different mapping is used.

External host needs to receive a packet before sending a UDP packet back to the internal host.

In practice: combinations between these behaviors.

Virtual Private Networks

Virtual Private Networks

VPN uses connections over an existing public network

Connection secured with encryption Host to Host Host to Gateway Gateway to Gateway

Virtual Private Networks

Virtual Private Networks

Encryption can be done at Application level. Transport level. Network level. Data link level.

Virtual Private NetworksVPN Technologies Application Level

Pretty Good Privacy Secure Shell (SSH)

Transport Level Secure Socket Layer

Does not protect the package, but its content. Typically runs at the application level of the OS, so OS does not need

to be changed. Network Level

IPSec Encrypts package itself. Encrypted package receives a new package header.

IPSec protects port address, but not destination address. OS need to be changed (but only once: Win2000, WinXP)

Data Link Layer 2 Tunneling Protocol addition to Point-to-Point protocol (PPP)

Encrypts packets on the data layer. L2TP (Layer 2 Tunneling)

Virtual Private Networks

Alternatives are dedicated point-to-point connections such as a private T1 line. Most secure. Most expensive. Takes time to set-up.